Quantum Cascade (QC) lasers are one of the key optical sources in the mid- and far-IR spectral range. In particular, QC lasers are well suited for applications in health and environment such as breath analysis, trace gas sensing, etc. For this reason, there is strong interest in the optimization of QC laser performance at room temperature with high power efficiency. There are many factors that influence the performance of QC lasers. For example, the electron transit time of many different Quantum Cascade lasers has been measured and compared to the calculated upper laser level lifetimes with and without taking into account interface roughness induced intersubband scattering. Improved designs are possible taking into account interface roughness induced intersubband scattering.